MUSEUM OF COMPARATIVE ZOOLOGY. 261 



?ven of the same cluster, mature at difFereut times. We must not sup- 

 pose, in studying the development of Agalma eggs, that the ova found 

 free in the water wei'e all cast at the same time. The only trustworthy 

 method of observation is to trace individual eggs into larvae and time 

 their development, which is a most difficult thing to accomplish success- 

 fully with these tender creatures. Even if we follow and time with 

 care the rate of growth in our glasses, it is a question whether we should 

 not make an allowance for retardation or acceleration of this time brought 

 about by changes in the teznperature of the water in which they are 

 placed in our aquaria. 



The disk formed at one pole of the egg by the epiblast, hypoblast, 

 and an intermediate transparent layer, may be called the primitive hy- 

 drophyllium or covering-scale, to distinguish it from others which are 

 later formed. In a stage following the last this body has assumed an 

 elevation upon the surface of the e^^ greater than formerly. As far as 

 its general outlines go, no great change has taken place in the larva with 

 advancinjr ace : but near the rim of the disk another minute elevation 

 in the walls of the epiblast has pushed itself up, which is destined later 

 to play an important part in the structure of the adult. This elevation 

 (^pn. CI/.), which at this time cannot be distinguished from a simple bud 

 such as any other organ of the Agalma body at first has, is the begin- 

 ning of the future float. It is a true bud, as already pointed out by 

 Metschnikoff. 



The accounts which Metschnikoff and Haeckel give of the origin of the 

 float in genera so nearly related as Agalma and Crystallodes are radically 

 different. In Crystallodes, according to Haeckel, the air-sac originates 

 from the primitive cavity as a bud. Speaking of changes on the sixth 

 day, he says : * " Die wichtigste Veranderung aber, welche am sechsten 

 Tage eintritt, ist die vollstiindige Abschniirung des Luftsackes von dem 

 Centralraum der Primitivhohle. Das Entoderm, welches die Wand des 

 Luftsackes bildet, und welches bisher an seiner Einmiindung in die 

 Centralhuhle unmittelbar iiberging nach oben in das Entoderm des 

 Deckstiick-Nahrcanals, nach vorn in das Entoderm des Polypiten, 

 wachset nun vollstiindig an dieser Stelle zusammen. Der Larvenkor- 

 per euthalt also nunmehr zwei vollstiindig getrennte und geschlossene, 

 mit FKissigkeit erfiillte Hohlen : die einfach rundliche oder langliche 

 runde Luftsackhohle, und die Centralhuhle, welche in vier Canale sich 

 verzweigt, in die Canale der beiden Knospen, des Deckstiicks und des 

 Polypiten. Das Entoderm, welches alle diese Hiihlraume auskleidet, 



* Op. cit., p. 58. 



